The present invention relates to a facsimile apparatus, printer or similar electrophotographic image forming apparatus and, more particularly, to a developing unit applicable to such an image forming apparatus for developing a latent image electrostatically formed on a photoconductive element.
A conventional developing unit for the above application includes a developing roller made up of a rotatable magnet roller and a rotatable developing sleeve coupled over the magnet roller. The magnet roller has a plurality of permanent magnets of opposite poles alternating with each other and each extending in the axial direction of the roller. The developing sleeve is a hollow cylinder formed of a nonmagnetic material. The developing roller is located to face a photoconductive element. A magnetic developer is deposited on the surface of the sleeve in the form of a magnet brush so as to develop a latent image electrostatically formed on the photoconductive element.
The above developing unit insures stable development free from the scattering of toner and the mixture of toner of different colors by preventing the magnetic adhesion of the developer to the developing sleeve from decreasing. For example, Japanese Patent Laid-Open Publication No. 7-219283 discloses a color image forming apparatus including a developing roller implemented by a cylindrical permanent magnet member having a plurality of magnetic poles thereon. A magnetic developer is deposited on the developing roller in the form of a layer whose thickness is smaller than a gap between the developing roller and the photoconductive element. While the developer is conveyed by the developing roller, an AC-biased DC voltage is applied between the photoconductive element and the developer so as to develop a latent image.
On the other hand, Japanese Patent Laid-Open Publication No. 8-328387 proposes a developing unit including a developing roller implemented by a cylindrical permanent magnet member having a plurality of magnetic poles thereon which are arranged at a pitch of 0.5 mm to 10 mm. The developing roller and a photoconductive element are rotated in opposite directions to each other. The developing roller and photoconductive element are respectively rotated at peripheral speeds of Vm (mm/s) and Vp (mm/s) satisfying a relation of 5.gtoreq.Vm/Vp.gtoreq.1.
The above conventional developing units each includes a casing formed with an opening at its bottom portion. The developing roller is disposed in the opening. A doctor is affixed to the lower edge of the opening for regulating the thickness of toner. Toner is stored in a toner storing section included in the casing and fed therefrom to be charged by friction acting between it and magnetic particles in the opening. Consequently, the toner is electrostatically deposited on the magnetic particles and conveyed by the developing roller to a position where the roller faces the photoconductive element.
Although a great amount of magnetic particles may exist in the casing, not all of them contribute to development or the charging of the toner. It follows that the toner cannot be sufficiently charged in a short period of time or to sufficiently develop a latent image formed on the drum. This brings about the contamination of a background and the scattering of the toner and thereby reduces the service life. Moreover, while fresh toner is replenished from the toner storing section, the toner short of charge due to insufficient friction acting between it and the magnetic particles in the opening is conveyed by the developer deposited on the developing roller. This also brings about background contamination and toner scattering.